1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a distributed feedback semiconductor laser used as a light source for optical fiber communications, etc.
2. Description of the Related Art
In recent years, as broadband communications and public telecommunication networks using optical fibers have become widely used, there has been an increasing need to transmit a large amount of information at low cost. To meet such a need, it is necessary to increase the amount of information that can be transmitted per unit time, that is, to increase the information transmission rate. Actually, the transmission rate has been progressively increased from 600 Mbps to 2.5 Gbps, to 10 Gbps.
Such an increase in the transmission rate of optical communications devices has led to an expansion in the market for optical communications networks for use not only in trunk systems but in access systems (offices, homes), requiring that the optical transceivers employ high-speed, high-efficiency, yet low-cost light emitting/receiving devices.
Semiconductor lasers externally modulated by an optical modulator have been conventionally used as semiconductor lasers for optical communications. However, when the transmitters and receivers are separated by a relatively small distance, as in access systems, directly modulated semiconductor lasers may be used, since they have a simple configuration and hence can be produced at low cost.
Waveguide ridge type distributed feedback laser diodes, buried heterostructure distributed feedback laser diodes, etc. are used as directly modulated semiconductor lasers. (A distributed feedback semiconductor laser diode is hereinafter referred to as a DFB-LD.)
One known waveguide ridge type semiconductor laser is formed of a Group III-V nitride semiconductor material and has a Fabry-Perot ridge stripe structure. This semiconductor laser has a problem in that if the carrier density of the active layer is increased to achieve high output power, the density varies in a lateral direction, these causing hole burning and kinking and hence limiting high-power operation.
To prevent this, a semiconductor laser has been disclosed in which: the resonator is divided into two portions by a line perpendicular to the length direction of the resonator; and the optical confinement factor of the active layer under the ridge stripe structure is lower on the front end face side than on the rear end face side.
In this example, the front end face side includes a p-type Al0.05 Ga0.95N cladding layer, while the rear end face side includes a p-type Al0.07Ga0.93N cladding layer. The p-type Al0.05Ga0.95N cladding layer has a higher refractive index than the p-type Al0.07Ga0.93N cladding layer. (See, e.g., paragraphs [0003], [0041], and [0044] and FIGS. 1 to 3 of Japanese Patent Laid-Open No. 2005-302843.)
Another known waveguide ridge type semiconductor laser is also formed of a Group III-V nitride semiconductor material and has a Fabry-Perot ridge stripe structure. In order to prevent formation of high-order horizontal transverse modes and occurrence of kinks as well as preventing degradation in laser characteristics, this semiconductor laser is formed such that the ridge portion includes two taper regions each tapering inwardly from a center portion of the resonator to a respective end of the resonator in the length direction, that is, the width of each taper region is reduced as a respective end of the resonator is approached. (See, e.g., paragraphs [0013], [0024], and [0025] and FIG. 1 of Japanese Patent Laid-Open No. 2000-357842.)
Further, one known buried heterostructure semiconductor laser device is constructed such that a Fabry-Perot semiconductor light-emitting element and a fiber grating form a resonator. This semiconductor laser device has a problem in that the distance between the fiber grating and the semiconductor light-emitting element is large, resulting in increased relative intensity noise due to resonance between the fiber grating and the light reflective film. This makes it difficult to achieve stable Raman amplification. To address this problem, a semiconductor laser device has been disclosed in which: an output side reflective film having a low light reflectance (1% or less) is formed on the light emitting end face; a reflective film having a high reflectance (70% or more) is formed on the light reflecting end face or the opposite end face; and the mesa stripe portion including an SCH-MQW active layer has a tapered shape, specifically, the portion of the mesa near the emitting side reflective film has a small width and the portion of the mesa near the high reflectance reflective film has a large width. This configuration allows the semiconductor laser device to generate laser light including two or more oscillation longitudinal modes. (See, e.g., paragraphs [0007] to [0009], [0033], and [0034] and FIG. 1 of Japanese Patent Laid-Open No. 2002-299759.)
Further, another known buried heterostructure semiconductor laser device has been proposed to increase the coupling efficiency between the semiconductor light-emitting element and the optical fiber. In order to emit light having a single-peaked pattern at a narrow emission angle without degrading operating characteristics such as the threshold current and slope efficiency, this semiconductor laser device is formed such that: the stripe-shaped active layer for generating laser light has continuously changing width throughout substantially the entire resonator region; and the width W1 of the active layer at the laser light emitting front end face and the width W2 of the active layer at the rear end face on the opposite side satisfy the relation: W2>W1. (See, e.g., paragraphs [0009], [0010], and [0017] and FIG. 2 of Japanese Patent Laid-Open No. 11-220220.)
In all conventional DFB lasers, whether waveguide ridge type or buried heterostructure type, the photon density distribution within the resonator is not uniform in the axial direction of the resonator or the optical waveguide direction.
In the case of a semiconductor laser with a diffraction grating having a phase shift structure, for example, the photon density distribution within the resonator is such that the photon density gradually increases from both end faces in the axial direction of the resonator toward the phase shift region and then dramatically increases to its maximum value within the phase shift region. In a semiconductor laser with a diffraction grating having no phase shift structure, on the other hand, the photon density may gradually increase from the emitting end face of the resonator toward the rear end face and then may dramatically increase at the rear end face portion, depending on the ‘end face phase’.
Incidentally, the relaxation oscillation frequency fr of a semiconductor laser, which represents the high speed response characteristics of the laser, is proportional to the square root of the product of the photon density S and the optical confinement factor G. In a conventional DFB-LD, since the optical confinement factor G is constant in the axial direction of the resonator, the distribution of the relaxation oscillation frequency fr in the axial direction of the resonator is substantially the same as the photon density distribution. As a result, the response speed within the resonator varies in the axial direction, that is, the resonator has a non-uniform response speed distribution in the axial direction.
Specifically, the response speed is high in a high photon density region and low in a low photon density region. Therefore, when a semiconductor laser having a non-uniform photon density distribution is subjected to direct modulation operation at high speed (e.g., 10 Gbps), the problem of distortion of the modulated light waveform, etc. arises.